The rapid growth in Internet access and Voice-Over-Internet Protocol (VOIP) services is putting increased pressure on service providers to find cost-effective means of carrying data traffic over their synchronous optical network (SONET) transport networks, originally designed for legacy circuit-switched, time division multiplexed (TDM) traffic. The same is true for service providers with synchronous digital hierarchy (SDH) networks, the predominant standard outside the U.S. It is to be appreciated that while we specifically use the term “SONET” herein, we intend for this to refer to both SONET and SDH networks.
To address the problem of carrying data traffic over SONETs, two architectures have been proposed. They are the Ethernet-Over-SONET (EOS) architecture and the Packet Ring (PR) architecture.
A SONET deployment typically consists of Add-Drop Multiplexer (ADM) nodes interconnected as a ring. Such a deployment follows the STS (synchronous transport signal) TDM hierarchy with ring capacities of STS-{12, 48, 192} (622 megabits per second, 2.5 gigabits per second, and 10 gigabits per second of bandwidth capacity, respectively), though next-generation SONET standards such as Virtual Concatenation (VCAT) and the Link Capacity Adjustment Scheme (LCAS) relax this strict hierarchy and allow capacities at the granularity of STS-1.
Mapping the data traffic from an Ethernet port directly into a SONET pipe leads to inefficient use of bandwidth since data traffic is bursty and the SONET pipe is provisioned at the peak rate. Consequently, in order to make effective use of the bandwidth, SONET ADMs are increasingly including Ethernet line cards (ELC) to perform statistical multiplexing of data traffic before inserting it into the ring. Such transport networks that are adapted to carry data traffic are referred to as packet-aware transports.
The two models, EOS and PR, differ in how they allocate transport bandwidth for the data traffic. FIGS. 1(a) and 1(b) show the respective schematics. The nodes S1-S5 (FIGS. 1(a) and 1(b)) are SONET Add-Drop Multiplexers and A1-A5 (FIG. 1(a)) and P1-P5 (FIG. 1(b)) are the packet processing nodes required in the Packet Ring and EOS models, respectively. Note that this packet processing can be done at the ELC cards mentioned above or in a separate node connected back-to-back to the SONET ADMs.
In the PR approach, a portion of the SONET ring is carved out and dedicated as a “virtual data ring” (VDR) for data traffic (e.g., STS-12 bandwidth from an STS-48 SONET ring). The data packets are processed and switched at every intermediate node along the path to the destination, as shown in FIG. 1(a) for demand D1 that gets switched at nodes A2, A3 and A4. It is to be understood that an intermediate node is a node in the subject path other than the source node and the destination node. The IEEE RPR standard (“IEEE 802.17 RPR,” IEEE Standard, 2004) follows this PR approach.
On the other hand, in the EOS approach, the data traffic originating at a node is aggregated on the ELC and mapped directly onto a SONET STS pipe of adequate granularity. This pipe is “expressed” through the SONET layer to the destination node with no packet processing at the intermediate nodes. Note that demand D1 does not get switched at intermediate node P3 in FIG. 1(b). This approach leads to the creation of a SONET pipe for each node pair that has a demand between them.
While the EOS approach and the PR approach each provide advantages, they each do not scale effectively to rapid data traffic growth. Thus, a packet-aware transport architecture that enhances data volume by scaling effectively to rapid data growth is desired.